Basic concepts of amplifier circuits and three basic connections

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Basic concepts of amplifier circuits and three basic connections [Copy link]

In order to make it easier for everyone to understand the amplifier circuit, let’s first introduce some basic concepts of the amplifier circuit.

(1) Input resistance and output resistance
An amplifier circuit can usually be equivalent to the circuit shown in Figure 2-4. The basis for this equivalence is: when the amplifier circuit is working, the input signal is sent to the input end of the amplifier circuit. For the input signal, the amplifier circuit is equivalent to a load resistor R i , and this resistor R i is called the input resistance of the amplifier circuit; after the amplifier circuit amplifies the input signal, it will output the signal to both ends of the load RL. Because the amplifier circuit has a signal output, for the load RL, the amplifier circuit is equivalent to a signal source with an internal resistance of R o and a voltage of U 2 , and the internal resistance R o here is called the output resistance of the amplifier circuit.

Figure 2-4 Amplifier circuit equivalent diagram

In the figure, U 1 is the signal source voltage, R 1 is the signal source internal resistance, and RL is the load. The part in the dotted line frame in the middle of the figure is the equivalent diagram of the amplifier circuit, U 2 is the amplified signal voltage, U i is the input voltage of the amplifier circuit, the voltage U o across the load RL is the output voltage of the amplifier circuit, the current I i flowing into the amplifier circuit is called the input current, the current I o flowing out of the amplifier circuit is called the output current, R i is the input resistance of the amplifier circuit, and R o is the output resistance of the amplifier circuit.
From the perspective of reducing the burden of the input signal source and increasing the output voltage of the amplifier circuit, it is better to have a larger input resistance R i, because when the internal resistance R 1 of the input signal source remains unchanged, a larger input resistance R i will reduce the I i current drawn by the amplifier circuit from the signal source, and at the same time, a relatively high U i voltage can be obtained at the input end of the amplifier circuit, so that the output voltage of the amplifier circuit after amplification is very high. If the output current I o of the amplifier circuit needs to be increased, it is better to have a smaller input resistance R i, because when the input resistance is small, the input current of the amplifier circuit is large, and the output current after amplification is relatively large.
The output resistance R o of the amplifier circuit should be as small as possible, because when the output resistance is small, the voltage and current consumed on the output resistance are very small, and the load RL can obtain a relatively large power. In other words, the smaller the output resistance of the amplifier circuit, the stronger the load capacity of the amplifier circuit.
(2) Amplification factor and gain
There are three types of amplification factors of the amplifier circuit.
① Voltage amplification factor. The voltage amplification factor refers to the ratio of the output voltage U o to the input voltage U i , represented by Au

② Current amplification factor. Current amplification factor refers to the ratio of output current Io to input current Ii, represented by Ai.

③ Power gain. Power gain refers to the ratio of output power P o to input power P i , expressed as AP

In practical applications, in order to facilitate calculation and expression, the logarithm of the gain factor is often used to express the amplification capability of the amplifier circuit. The value obtained in this way is called gain, and the unit of gain is decibel (dB). The larger the gain, the stronger the amplification capability of the circuit.

For example, when the voltage amplification factor of the amplifier circuit is 100 times and 10,000 times respectively, its voltage gain is 40dB and 80dB respectively.

Three basic connections of amplifier circuits

According to the different connection methods of the transistor in the circuit, there are three basic connection methods for the amplifier circuit: common emitter connection, common base connection and common collector connection. The three basic connection methods of the amplifier circuit are shown in Figure 2-5.

Figure 2-5 Three basic connections for amplifier circuits

The three basic connection methods of the amplifier circuit can be analyzed from the following aspects.
(1) Whether it has the ability to amplify
As mentioned earlier, to determine whether the transistor circuit has the ability to amplify, it can generally be determined by analyzing whether the Ib, Ic, and Ie currents of the transistors in the circuit have a complete path. If there is a complete path, it means that the amplifier circuit has the ability to amplify. In the circuit shown in Figure 2-5, the analysis of the Ib, Ic, and Ie currents of the transistors in the three basic connection methods is as follows. The
Ib, Ic, and Ie current paths of the transistors in the common emitter connection circuit:

The current paths of Ib, Ic and Ie of transistors in the common base connection circuit are:

The current paths of Ib, Ic and Ie of transistors in the common collector connection circuit are:

From the above analysis, it can be seen that the currents of transistors Ib, Ic, and Ie in the three basic connection circuits all have complete paths, so they all have amplification capabilities.
(2) Shared electrode form
An amplifier circuit should have input and output terminals. In order for the AC signals at the input and output terminals to have their own loops, the input and output terminals should each have two electrodes, while the transistor has only three electrodes, so one electrode will be shared by the input and output terminals.
When analyzing the amplifier circuit, in order to understand the processing of the AC signal of the amplifier circuit, it is necessary to draw its AC equivalent diagram. When drawing the AC equivalent diagram, DC is not considered. The following two points should be mastered when drawing the AC equivalent diagram.
① The internal resistance of the power supply is very small, and it can be regarded as a short circuit for AC signals. That is, for AC signals, the positive and negative poles of the power supply are equivalent to a short circuit, so when drawing the AC equivalent diagram, the positive and negative poles of the power supply should be connected with wires.
② The coupling capacitor and bypass capacitor in the circuit have relatively large capacity, which has little resistance to AC signals and can also be regarded as a short circuit. When drawing the AC equivalent diagram, large-capacity capacitors should be replaced by wires.
According to the above principles, the AC equivalent diagram of the common emitter connection amplifier circuit shown in Figure 2-5(a) can be drawn according to the method shown in Figure 2-6.

The same method can be used to draw the AC equivalent diagrams of the other two basic connection amplifier circuits. The AC equivalent diagrams of the three basic connection amplifier circuits are shown in Figure 2-7.

Figure 2-7 AC equivalent diagrams of three basic connection amplifier circuits

In the circuit shown in Figure 2-7(a), the base is the input terminal, the collector is the output terminal, and the emitter is the common electrode of the input and output circuits. This amplifier circuit is called a common emitter amplifier circuit.
In the circuit shown in Figure 2-7(b), the emitter is the input terminal, the collector is the output terminal, and the base is the common electrode of the input and output circuits. This amplifier circuit is called a common base amplifier circuit.
In the circuit shown in Figure 2-7(c), the base is the input terminal, the emitter is the output terminal, and the collector is the common electrode of the input and output circuits. This amplifier circuit is called a common collector amplifier circuit.
(3) Characteristics of the three basic connection amplifier circuits
The characteristics of the three basic connection amplifier circuits are shown in Table 2-1.

Table 2-1 Characteristics of three basic connection amplifier circuits